feat(tvix): implement string interpolation

This adds a new instruction which assembles an interpolated string
from a specified number of fragments, which are already going to be
located on the stack in the right position.

This will raise a type error if any of the fragments do not evaluate
to a string.

Change-Id: I5756248fa3e9fcc3d063c14db40b332f7e20a588
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6098
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/opcode.rs

@@ -35,4 +35,7 @@ pub enum OpCode {
 
     // Lists
     OpList(usize),
+
+    // Strings
+    OpInterpolate(usize),
 }